This invention relates to improved methods for extracting cobalt from hydroprocessing catalysts.
In processing hydrocarbonaceous feedstocks, the feedstocks are frequently contacted with hydroprocessing catalysts and molecular hydrogen to hydrogenate various molecules in the feedstock, producing products with different properties than the original feedstock. The catalysts contacted generally contain Group VI-B catalytic metals and a Group VIII transition metal supported on a solid inorganic oxide support. Typical Group VI-B metals include molybdenum and tungsten. A particularly common Group VIII metal component is cobalt. Inorganic supports frequently include alumina or alumina and silica mixtures, although they may be clays or mixtures of other inorganic oxides including titanium, magnesia, boria and the like. As these catalysts are used, they become less active until they reach a point of deactivity where they must be replaced with new catalysts to maintain the desired product quality. Such deactivated catalysts are herein defined as "spent catalysts."
The spent catalyst which is removed from service still contains the valuable catalytic metals, in particular molybdenum and cobalt. It may as well contain other metals, particularly if the catalyst has been used to process crude oils or residual oils. In particular, it may contain amounts of iron, nickel and vanadium removed from the feedstock. It is desirable that the catalytic metals on the catalyst particles be removed and recycled to make new catalyst particles that have renewed activity. To do this, the metals must be removed from the support and recycled.
A variety of processes are known to remove metals from spent catalysts. One particularly good process is leaching the spent catalyst with any of a variety of liquids, primarily aqueous solutions containing various leaching agents. One particularly refractory metal towards typical leaching conditions used during the leaching of hydroprocessing catalysts is cobalt. Cobalt yields are typically quite low, although the catalyst may contain up to several weight percent cobalt.
Cobalt is a particularly valuable strategic metal and is a frequent component of hydroprocessing catalysts where it is used in relatively large amounts. A steady source of cobalt is therefore of great interest to catalyst manufacturers and the recycling of cobalt from spent hydroprocessing catalysts, which contain other metals as well, is therefore highly attractive as a source of cobalt for the manufacture of new hydroprocessing catalysts. Unfortunately, cobalt refractory towards conventional leaching techniques that extract other metals on the catalyst in high yield.
It has been discovered that cobalt containing hydroprocessing catalysts can be leached with an aqueous solution of cyanide to improved yields of cobalt recovered, after the particles have been leached by an aqueous ammonia/ammonium salt leach.